The term "casual game" was coined because the people who develop games wanted a way to distinguish games that are not to be taken seriously from those games played on consoles. It depends which side of the fence you sit on to determine if the term is positive or negative. Games that are considered "hard-core" games are typically delivered on game consoles or PCs and require large budgets to develop and deliver. Casual games are quite the opposite: they typically are designed, developed, and delivered by very small teams on very small budgets. Therefore, casual games are often considered not real games by the people who make real games. Conversely, players who play games really don't care. What's important is if the game is fun.
So what makes a game "casual"? A casual game would be characterized by short game play sessions with very simple controls (point-and-click) and leveraging familiar game play mechanics. Anyone can play. There are no special skills required or advanced knowledge. The point is, they are simple games both from a user/player experience and from development complexity.
There is some heated argument what game would be considered the first casual game. I played Pong and Star Trek in the late 1970's and by today's standard they are causal games but they certainly weren't then.
Some claim Pacman or Tetris are casual, but I don't agree. Minesweeper and Solitaire on Windows are really the first casual games. Simple to pick up, anyone can play. I consider Bejeweled to be the first game that fits the definition of casual game. I first played it on AOL over dial-up network and it was the classic simple, point-and-click, short game session mechanic that defines casual game.
Bejeweled was available to play online and I think that is what made the difference. It started a craze, and what followed was a gold rush to build out website dedicated to playing games online.
When I started working at Skyworks in 1999 I dedicated my career to making these kinds of simple games that are played online. I had experimented with several game projects in the past, including Atari 800, Windows and DirectX, and text-based games written in BASIC. I had a passion for this. We were responsible for Candystand, one of the very first website dedicated to playing games online. We experimented with several technologies at the time: Java, C/C++, Flash, and Director. At that point in time Director and Shockwave was the only technology capable of delivering near-console experience game play on the web. Nothing else came close. My first completed game project was a Mah-jongg game influenced by Microsoft Taipei.
I was hooked on making games. Since my first release in 1999 I have made more games than I can count, well over 40 titles. The experience never gets old and I continue to experiment and try out new ideas. And having written this I am now yearning to get back to coding a game!